1. Field of the Invention
The present invention relates to a GPS reception ratio detecting apparatus, and also to a portable type distance/speed meter capable of measuring both a travel distance and a travel speed, equipped with this GPS reception ratio detecting apparatus. The GPS reception ratio detecting apparatus detects a GPS reception ratio indicative of a ratio at which effective GPS electromagnetic waves are received by a GPS receiver.
2. Description of the Related Art
In the GPS (Global Positioning System), 24 sets of the GPS satellites orbit on 6 sets of orbit courses located at an inclined angle of 55 degrees at a distance of approximately 20,200 km on the earth, and travels for approximately 12 hours per one turn. While navigation data required for positioning, transmitted from more than 3 GPS satellites under the most receivable condition are received by a GPS receiver, positioning calculations are carried out by measuring propagation delay time of these navigation data so as to determine travel direction/present position of a user.
In this GPS, two different frequencies "L1 (=1.57542 GHz)" and "L2 (=1.22760 GHz)" are prepared for the transmission frequencies of the GPS satellites. Since the C/A code (namely commercial-purpose code being free-opened) is transmitted at the frequency of 1.57542 GHz (equal to GPS transmission frequency "L1"), one GPS transmission frequency "L1" is utilized in general-purpose positioning operation. It should be understood that the GPS signal having this frequency "L1" is modulated in the PSK (Phase Shift Keying) modulating method by using the pseudonoise code, and then the PSK-modulated GPS signal is transmitted by way of the spread spectrum communication system. This pseudonoise code corresponds to the synthesized wave made from the C/A code used to discriminate the desirable GPS satellite from all of the GPS satellites, and also the navigation data such as the GPS satellite orbit, the GPS satellite orbit information, and the time information.
FIG. 6 is a schematic block diagram representing an arrangement of a GPS receiver 200 capable of receiving a GPS electromagnetic wave (namely, GPS signal having frequency of "L1 (=1.57542 GHz)") transmitted from a GPS satellite. As shown in FIG. 6, the GPS receiver 200 is arranged by a reception antenna 201, an L-band amplifying circuit 202, a down-converter circuit 203, a voltage comparing circuit 204, a message decrypting circuit 205, and a positioning calculating circuit 206. The reception antenna 201 receives GPS electromagnetic waves transmitted from the GPS satellites. The L-band amplifying circuit 202 amplifies a GPS signal having an L-band frequency among the received GPS signals. The down-converter circuit 203 performs a down-converting operation of the amplified GPS signal by multiplying this received GPS signal by a signal produced from a local oscillating circuit 107. The voltage comparing circuit 204 digitally converts the GPS signal down-converted by the down-converter circuit 203 into a digital GPS signal. In the message decrypting circuit 205, the digital GPS signal inputted from the voltage comparing circuit 204 is multiplied by a C/A code generated from a C/A code generating circuit 208 so as to acquire both navigation data and carrier wave phase information corresponding to a pseudodistance. The positioning calculating circuit 206 calculates positioning data by using both the navigation data and the carrier wave phase information, which are entered from the message decrypting circuit 205. It should also be noted that the local oscillating circuit 107 corresponds to such a circuit capable of producing a signal used to convert a received GPS signal into another signal having a desirable frequency.
Next, reception operation of this GPS receiver 200 will now be described. In FIG. 6, the L-band amplifying circuit 202 selectively first amplifies the GPS signal having the frequency of 1.57542 GHz received by the reception antenna 201. The GPS signal amplified in the L-band amplifying circuit 202 is entered into the down-converter circuit 203. This down-converter circuit 203 converts this entered GPS signal into a first IF (intermediate frequency) signal having a frequency of from several tens of MHz to 200 MHz by using the local oscillation signal produced from the local oscillating circuit 107, and furthermore, converts this first IF signal into a second IF signal having a frequency on the order of from 2 MHz to 5 MHz. Then, the voltage comparing circuit 204 enters thereinto this second IF signal so as to digitally convert the second IF signal into the digital GPS signal by employing a clock signal having a frequency several times higher than the frequency of this entered second IF signal. In this circuit, this digitally-converted GPS signal will constitute spectrum-spread data (digital signal).
This spectrum-spread data outputted from the voltage comparing circuit 204 is entered into the message decrypting circuit 205. Then, this message decrypting circuit 205 reverse-spreads the C/A code produced from the C/A code generating circuit 208 to the entered digital signal so as to acquire both the navigation data and the carrier wave phase information corresponding to the pseudodistance. The C/A code implies the pseudonoise code identical to that of the GPS satellite.
The above-explained reception operation is carried out with respect to the respective GPS satellites in this GPS receiver 200. Normally, the message decrypting circuit 205 of the GPS receiver 200 may acquire the navigation data and also the carrier wave phase information of 4 sets of the GPS satellites, and then the positioning calculating circuit 206 acquires the positioning data (speed, present position, time information etc.) based upon the acquired navigation data/carrier wave phase information. The positioning data acquired by the positioning calculating circuit 206 is outputted to a CPU (not shown) for controlling the overall reception operation of this GPS receiver 200, or externally outputted as a digital signal. Such a GPS receiver is utilized as a car navigation system by combining positional information of GPS with map information produced from a CD-ROM.
On the other hand, the above-explained GPS receiver 200 is realized in the form of such a portable type GPS receiving apparatus capable of measuring travel speeds/travel distances of persons, since the GPS receiver 200 may be supplied as a digital ASIC (Application Specific IC) due to current technical progresses in semiconductor fields. This portable type GPS receiving apparatus calculates the travel distance and the travel speed of the user based upon the positioning data acquired by employing the GPS receiver 200, and then displays both the travel distance and the travel speed.
However, there are many possibilities that the above-explained GPS receiver 200 cannot receive the GPS electromagnetic waves, because the reception of these GPS electromagnetic waves is disturbed by various disturbing objects, for example, bottom places among buildings and places inside tunnels, and/or when one GPS satellite captured by this GPS receiver 200 is switched to another GPS satellite. As a result, the GPS receiver 200 can hardly acquire the positioning data. Such an unreceivable condition of the GPS electromagnetic waves may be avoided by restricting the utilization environment of this GPS receiver 200. However, this environmental restriction cannot sufficiently achieve the advantages specific to the GPS capable of acquiring global geographical information.
On the other hand, in the above-explained portable type GPS receiving apparatus, both the travel speed and the travel distance when the user walks or runs are calculated by employing the GPS receiver 200. As a consequence, in such a case that the reception of these GPS electromagnetic waves cannot be carried out, the correct GPS calculation results cannot be obtained. More specifically, since a travel trace of a person normally contains a large number of bending portions and curved portions, in such a case that the positional information is extracted from the positioning data acquired from the GPS receiver 200 and then the positional change thereof is calculated as the travel distance, the below-mentioned problem may occur. That is, under such a GPS electromagnetic waves unreceivable condition, a so-called "positional jump" occurs many times, so that a large number of errors are involved in the calculated travel distances. Also, as to the travel speeds, since this travel speed is calculated based upon this erroneous travel distance, a similar problem may occur.